The present invention relates to an obstacle detector for use in an automotive vehicle which detects any obstacle, or any other vehicle whether moving or parked, present on the road on which the vehicle with the detector runs by transmitting a light signal against the obstacle or other vehicle and receiving the reflected light from the obstacle or other vehicle.
It is necessary to provide automotive vehicles with obstacle detectors at the front or rear, or both, of the vehicles to detect any obstacle, or any other vehicle whether moving or parked, likely to be present on the road on which the vehicle with the detector travels so as to avoid collision with the obstacle or the other vehicle by generating a warning signal to the driver or by stopping the vehicle automatically in an emergency.
If a light beam is used for detecting purposes, it may be adversely affected by solar energy in the daytime, or by the light from the headlights of other vehicles or streetlamps at night so that erroneous detection may occur.
It has been shown in the internal magazine "FUJITSU" Vol. 27, No. 6, 1976 published by FUJITSU LIMITED in Japan that due to the humidity contained in the atmosphere, the transmission of light through the atmosphere is very low in the range of 6.5.+-.1 microns.
The transmittance of light through the front glass of vehicle headlights or streetlamps is very low for light having wavelengths in excess of approximately 5 microns. The transmittance of light through other general types of glass is also very low except for the following ranges: 0.28 to 4.5 microns for Borosilicate (Pyrex) glass; 0.21 to 5.0 microns for shrunk (or Vycor) glass; 0.12 to 4.5 microns for Silica (SiO.sub.2) glass (quartz); and 0.21 to 4.5 microns for Silica (SiO.sub.2) glass (fused silica). Thus, light passing through the front glass of a light source has a wavelength of at most 5 microns so that the intensity of light with wavelengths larger than 5 microns is almost zero.